The present invention relates to a method for producing electrodes in which a porous anodic oxide is deposited by anodic oxidation on surfaces of a plurality of finely patterned metal wiring lines of such devices as semiconductor devices and active liquid crystal electro-optical devices in such a manner that the thickness of an anodic oxide film formed on at least one wiring line is different from the thicknesses of anodic oxide films formed on the other wiring lines.
Conventionally, the following method is employed to form porous anodic oxide films on respective fine wiring lines of several microns to several tens of microns in width and several microns to several hundred of microns in interval with the degree of oxidation varied for the respective wiring lines. The following example is directed to a case where anodic oxide films are formed on two wiring lines at different thicknesses.
As schematically shown in FIG. 2, a first wiring line 3 and a second wiring line 4 that are made of aluminum and provided on an insulative base 1 and a cathode electrode 2 are immersed in a forming solution (electrolytic solution for anodic oxidation; not shown) that is contained in a vessel. The forming solution may be a 3% aqueous solution of an oxalic acid, an aqueous solution of a sulfuric acid, etc.
A power supply 5 is so provided that a positive potential is applied to the first wiring line 3 and the second wiring line 4 and a negative potential to the cathode electrode 2. The first wiring line 3 and the second wiring line 4 are connected to the power supply 5 via respective switches 6 and 7, which independently perform on/off-control of currents.
The base 1 and the cathode 2 are spaced from each other by 15 to 50 mm. When anodic oxidation is performed, the switches 6 and 7 are turned on to supply 5 to 30 V, in which case the electric field strength is 0.1 to 2 V/mm.
The thickness of an oxide film deposited is proportional to the forming period (voltage application period). Therefore, for example, to make an anodic oxide film deposited on the second wiring line 4 two times thicker than that deposited on the first wiring line 3, the switches 6 and 7 are so controlled that a constant voltage is applied to the first wiring line 3 for 30 minutes and the second wiring line 4 for 60 minutes.
Similarly, in the case of three or more wiring lines, they may be provided with oxide films of two or more, or three or more different thicknesses only by changing the voltage application period.
However, in the above method, it is sometimes the case that the insulation between wiring lines, which is secured before and after anodic oxidation, becomes worse during the anodic oxidation, to cause current leakage. In this case, a current continues to flow through a wiring line even after an associated switch has been turned off to form a thinner oxide film thereon. As a result, this wiring line is provided with an oxide film whose thickness is almost the same as the thickness of other wiring lines.
The above current leakage is considered to occur in the following manner. Impurities, such as dust, minute conductive substances and ionized substances, included in a forming solution are attracted by an electric field developing between the anode and cathode that are given a voltage from the power supply 5, and possibly attached to those electrodes. The impurities produce an electrically conductive state between the wiring lines 3 and 4 that are arranged at a very small interval, i.e., and worsen the insulation between the wiring lines 3 and 4.